1. Technical Field
The invention relates to a method and apparatus for reducing the NO.sub.x output of slotted roof fired boilers. More particularly, the invention relates to a method and apparatus which is retrofitted to existing multiple-intertube pulverized-coal burners to reduce the NO.sub.x output of the burners. Even more particularly, the invention relates to a method and apparatus using air staging ports and air baffles within the windbox of the multiple-intertube pulverized-coal burners to reduce the NO.sub.x output of slotted roof fired boilers to meet the proposed standards of the Clean Air Act.
2. Background Information
Electric power plants require some form of initial energy to produce the steam that powers the generators to produce electricity. One type of initial energy is heat produced from the burning of pulverized coal in a multiple-intertube pulverized-coal burner. The pulverized coal or fuel is carried into the burners of a slotted roof fired boiler by a primary air flow. A quantity of secondary air is then provided at the burner tips and mixes with the primary air/fuel mixture. The secondary air supplies about 80% of the total air required for combustion while the primary air provides the remaining 20%. Combustion occurs and the boilers then produce the steam which is subsequently converted into electrical energy by the steam driven turbines. Conventionally, the secondary air is introduced through a windbox and distributed to the individual burners. This allows for sufficient oxygen atoms to be present in the burners to burn the coal's volatile (hydrocarbon) content and the fuel's fixed carbon atoms.
During the combustion process, nitrogen trapped in the coal particle is released and, in the presence of excess oxygen, can combine to form nitric oxide (NO), or nitrogen dioxide (NO.sub.2), both of which are classified as pollutants. When both gases are referenced simultaneously they are referred to as NO.sub.x. Similarly, under the intense thermal environment of a furnace or combustion chamber, atmospheric nitrogen can disassociate into nitrogen atoms which can then combine with excess oxygen to form NO.sub.x. The Clean Air Act addresses the amount of these gases that these burners may emit and new standards must be complied with by the year 2000. The method and apparatus of the present invention reduces the NO.sub.x emissions from existing multiple-intertube pulverized-coal burners in order to meet the requirements of the Clean Air Act.
Many prior art apparatuses have attempted to reduce the level of NO.sub.x emissions from various coal burning boilers in an effort to meet the government standards set forth in the Clean Air Act and to prevent the discharge of such gases into the surrounding atmosphere.
U.S. Pat. No. 5,417,564 shows a method and apparatus which alters the firing pattern of the burners. A panel is cut-out of the furnace wall and reinstalled in a reverse orientation to provide a greater spacing or staging distance between first and second burners.
U.S. Pat. No. 5,329,866 shows a burner and port combination which includes a throat, a burner nozzle positioned at a central area of the throat, a secondary air tube positioned laterally adjacent the burner nozzle and a plurality of vanes positioned at an upper portion of the throat above the burner nozzle.
U.S. Pat. No. 5,199,355 shows a low NO.sub.x short flame burner which includes a central nozzle pipe having an inner surface with a portion which diverges outwardly. An axially movable plug positioned within the nozzle pipe is axially moved to increase and decrease the velocity of a fuel and air mixture thus reducing the formation of NO.sub.x and the length of the flame produced by the burner.
Although these prior art apparatuses may be adequate for the purpose for which they were intended, these prior art devices fail to solve the emission problems of existing slotted roof fired boilers. Thus, the need exists for a method and apparatus for reducing the NO.sub.x emissions from slotted roof-fired boilers.
The method and apparatus of the present invention reduces the level of NO.sub.x emissions in these slotted roof fired boilers to meet the proposed new standards of the Clean Air Act while minimizing any detrimental impact on combustion efficiency. The method and apparatus of the present invention uses an internal two stage combustion process to reduce the NO.sub.x emissions which uses commercially available air flow instrumentation to control the amount of air inputs into the burners, which provides better uniformity of flame chemistry across the unit and over the full load range than the previous methods and apparatuses, and which has a relatively low installation cost. The use of interjectory air produces a reduction in the thermal NO.sub.x by enlarging the flame envelope and internal burner air staging produces a reduction in both the thermal and fuel NO.sub.x by reducing the available oxygen at the root of the flame where rapid devolatization of the pulverized coal occurs and by reducing turbulence and mixing of the combustion air and fuel at the root of the flame. There is no such method and apparatus of which we are aware which accomplishes these results.